The invention relates to a graded layer or zone at the surface of fused silica for reducing its ultraviolet transmission and which is also useful to inhibit sodium ion diffusion through the walls of silica envelopes.
The ultraviolet transmissivity of fused silica extends well below 180 nanometers. Thus fused silica envelopes transmit both the 184.9 nm. line and the 253.7 nm. line produced by the mercury discharge. The former produces ozone in air which has a germicidal effect and a noticeable odor whereas the latter has an erythemal effect on the skin and is harmful to the eyes. In the usual jacketed mercury or metal halide lamp wherein the fused silica arc tube is enclosed in an outer glass envelope, the ultraviolet emission is contained by the outer envelope. However with unjacketed lamps where the 184.9 or 253.7 nm. radiation or other ultraviolet radiation is objectionable, recourse must be had to envelopes made of filter-type fused silica. Such fused silica is available from Quartz and Silice Company, Paris, France under the names Germisil and Heliosil. Germisil effectively absorbs radiation of wavelength below 250 nm. but is transparent to actinic and bactericidal rays. Heliosil effectively stops all radiation below 280 nm. and may be used to transmit only the near ultraviolet which produces skin tanning without giving rise to erythematous action. These fused silicas are described by the manufacturer as consisting of 99.97% SiO.sub.2 with a slight addition of titanium. While perfectly satisfactory for the intended purpose, lamp envelopes made of these special fused silicas are very expensive.
Metal halide lamps for general illumination contain a filling of mercury and light-emitting metals including sodium in the form of halides, commonly the iodide, in fused silica arc tubes enclosed in outer glass envelopes. A problem encountered during operation of these lamps is the slow passage of sodium from the hot arc plasma through the fused silica wall into the cooler region between the arc tube and the outer envelope. The lost sodium can no longer contribute its characteristic emission so that the light output gradually diminishes and the color shifts from white towards blue. Loss of sodium also causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained by the ballast at which point the life of the lamp is ended.
In the past several different types of coatings on fused silica have been tried to reduce sodium ion diffusion. One proposed barrier layer comprised a layer of zirconium oxide upon the inner surface of the arc tube to inhibit sodium diffusion, and a second layer of aluminum oxide over the zirconium oxide to protect the zirconium oxide from the attack of the arc stream. It has also been proposed to provide a radiation absorbing glaze on the outer or inner surface of the quartz arc tube to absorb ultraviolet light. It was reasoned that hydrogen absorbed in the glass of the outer jacket is released by the photochemical effect of ultraviolet light on the glass. Such hydrogen would then diffuse into the hot fused silica arc tube, where it shortens life, makes the lamp more difficult to start, and increases the voltage necessary to sustain the arc discharge. Such coatings have not been sufficiently successful for commercial adoption.
The object of the invention is to provide an improved layer or surface zone in fused silica which will reduce the ultraviolet transmission of the silica and also lower its sodium ion conductivity, and a novel method for forming such layer or zone.